Packin’ it on. Helium atoms (in purple regions) stick to a nitrous oxide molecule (blue and red)–first around the middle, and then at either end–and make it harder to spin. Add even more helium atoms, however, and they slide around the cluster without resistance.

Chilly helium is surely the slickest of liquids. When liquid helium is cooled to within a couple of degrees of absolute zero, it flows without any friction or viscosity. Known as superfluidity, the phenomenon allows the liquid to flow endlessly around a loop-shaped pipe without slowing down. Even a miniscule dollop of the stuff–just seven atoms–can produce resistance-free motion, a team of chemists and physicists reports in the 17 October PRL.

Superfluidity in liquid helium arises when, at temperatures below 2.17 kelvin, some of the atoms gang up to form a macroscopic quantum wave, so part of the liquid flows without any drag or resistance. In 1946, Russian physicist Elevter Andronikashvili demonstrated resistance-free flow by immersing a stack of thin metal disks in helium and twisting it back and forth about its axis. Ordinarily, the helium would stick between the closely spaced plates just as honey sticks between the fins of a honey dipper. The helium would increase the stack’s inertia, making it harder to twist. But when the cooling helium began to turn into a superfluid, some of it flowed freely between the plates, so the inertia of the stack suddenly began to drop.

In recent years, researchers have applied a souped-up version of Andronikashvili’s classic experiment to ever smaller amounts of helium. Instead of twisting stacks of plates, researchers have used infrared light and microwaves to spin individual molecules of carbonyl sulfide (OCS) embedded in tiny droplets of helium. By studying the spectrum of absorbed wavelengths, researchers have found that the molecules spin freely in “nanodroplets” containing several thousand atoms, suggesting that these droplets are superfluid. On the other hand, in miniscule clusters of eight or fewer helium atoms, all the helium appears to stick to the OCS molecule, giving it even more inertia than it has in the much bigger droplets, as physicist Robert McKellar of the National Research Council of Canada in Ottawa, chemist Wolfgang Jäger of the University of Alberta in Edmonton, and colleagues found last year.

Now the Canadian team has traced the crossover from sticking to slipping by spinning a different molecule–nitrous oxide, or N2O. Taking the same tack as in their experiments with OCS, the researchers forced a mixture of helium and nitrous oxide through a very cold nozzle to produce clusters containing a molecule and a handful of helium atoms. They zapped the clusters with infrared light and microwaves to make the molecules vibrate and spin. Clusters with different numbers of helium atoms absorbed radiation at slightly different wavelengths, and the researchers carefully disentangled the intricate and overlapping spectra to determine how easily the molecule spun in each size cluster.

Adding up to six helium atoms drove the molecule’s inertia up, they report. Adding a few more atoms, however, caused its inertia to level off and begin to fall. These additional atoms slide freely around the molecule and even loosen up some of the underlying helium that would otherwise hold fast to it, the researchers say.

“This turnaround was predicted, but it is very important to observe it,” says physicist Saverio Moroni of the University of Rome, “La Sapienza.” Kevin Lehmann, a chemist at Princeton University in New Jersey, says the experiment is a technological tour de force. “Previously, people could do the experiments with one or two helium atoms, or tens of thousands of them, but nothing in between,” he says. “These guys can do three, four, five, six, seven, eight, nine, ten, and can assign them all,” to specific spectral wavelengths.